Ligands Mediate Anion Exchange between Colloidal Lead-Halide Perovskite Nanocrystals

The soft lattice of lead-halide perovskite nanocrystals (NCs) allows tuning their optoelectronic characteristics via anion exchange by introducing halide salts to a solution of perovskite NCs. Similarly, cross-anion exchange can occur upon mixing NCs of different perovskite halides. This process, though, is detrimental for applications requiring perovskite NCs with different halides in close proximity. We study the effects of various stabilizing surface ligands on the kinetics of the cross-anion exchange reaction, comparing zwitterionic and ionic ligands. The kinetic analysis, inspired by the “cage effect” for solution reactions, showcases a mechanism where the surface capping ligands act as anion carriers that diffuse to the NC surface, forming an encounter pair enclosed by the surrounding ligands that initiates the anion exchange process. The zwitterionic ligands considerably slow down the cross-anion exchange process, and while they do not fully inhibit it, they confer improved stability alongside enhanced solubility relevant for various applications.

iii. TOP-Br2 0.5 M in toluene: TOP (6mL, 13 mmol) and bromine (0.6 mL, 11.5 mmol) were mixed under inert atmosphere. Once the reaction was complete and cooled to room temperature, the TOP-Br2 was dissolved in toluene (18.7 mL).
iv. TOP-Cl2 0.5 M in toluene: solution was centrifuged at 29500g (g is the earth gravity) for 10 minutes and the precipitate was dispersed in 10 mL of toluene and 6 centrifuged again at 29500g (g is the earth gravity) for 10 minutes to precipitate the fraction of NCs that are not colloidally stable.
vii. CsPbBr3 nanocrystals with C3-ASC18 as a ligand: CsPbBr3 NCs were synthesized by dissolving Cs-oleate (4 mL, 1. . After the last precipitation, NCs were dispersed in 2 mL of toluene and centrifuged at 29500 g for 1 minute to remove any nondispersed residue.
x. CsPbCl3 nanocrystals with lecithin as a ligand: CsPbCl3 NCs were synthesized by dissolving Cs-oleate (4 mL, 1.6 mmol), Pb-oleate (5 mL, 2.5 mmol) and lecithin (0.65 g, ca. 0.9 mmol) in ODE (5 mL) and heating the mixture under vacuum to 130 °C, whereupon the atmosphere was changed to argon and TOP-Cl2 in toluene (5 mL, 5 mmol of Br) was injected. The reaction was cooled immediately by an ice bath. The crude solution was precipitated by the addition of 2 volumetric equivalents of acetone, followed by the centrifugation at 29500g (g is the earth gravity) for 10 minutes. The precipitated fraction was dispersed in 10 mL of toluene and then washed three more times. Each time the solution was mixed with two volumetric equivalents of acetone and centrifuged at 29500 g for 1 minute, and subsequently dispersed in the progressively smaller amounts of the solvent (5mL for the second cycle, 2.5 mL for the third cycle). After the last precipitation, NCs were dispersed in 2 mL of toluene and centrifuged at 29500 g for 1 minute to remove any nondispersed residue. 9 Sizes of the nanocrystals (NCs): OA/OLA: • CsPbCl3 -6±1 nm (62 NCs), CsPbBr3 -7±1 (61 NCs) -kinetic experiments.
• CsPbCl3 -8.7±0.9 nm (60 NCs), CsPbBr3 -9.0±0.8 (60 NCs) -size dependency. C3-ASC18: • CsPbCl3 -10±1 nm (61 NCs), CsPbBr3 -9±1 (61 NCs) -kinetic experiments. The extracted rate constant of anion exchange in all three ratios was 0.002±0.001 s -1 at 25 °C. This independence on the initial concentration ratio is fully consistent with the pseudo first-order kinetics. Considering this and the significantly lower PL quantum yield of CsPbCl3 relative to CsPbBr3 we chose to use a ratio of Cl:Br=8.5:1, in which we were able to detect and reliably analyze also the high energy peak with chloride enriched NCs.

Extinction coefficient
Nanocrystal concentrations in solution were determined using their respective extinction coefficients. For CsPbBr3 the published value was used. 5

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The extinction coefficient for CsPbCl3 NCs was estimated using a combination of absorption spectroscopy and TGA. The absorption spectrum was recorded. Then, the same amount was used in order to determine the mass of the inorganic content by TGA measurements.
The absorption spectrum is very variable with size differences due to the large band gap energy and interference of organic materials at energies sufficiently remote of the excitonic transition. The received extinction coefficient is hence a rough estimation and should not be generalized to different NC sizes. Where is the bulk density of CsPbX3.

Halide number in NC and
The number of halides is three times N.
b. Cs surface sites: The number of Cs surface sites is the ratio between the surface area of the NC and the area of one face of the unit cell. For simplicity the lattice parameters of the cubic bulk phases were used. 6 The lead bromide bond lengths do not vary much between the different phases such that this is a sufficient approximation for our purposes.

Sample preparation for optical kinetic experiments
The kinetic experiments were conducted in a quartz cuvette containing 2 ml of CsPbCl3 in toluene with O.D. of 1.05.
The kinetic experiment started by swiftly injecting 15 µl of CsPbBr3 in toluene, which contained 0.3 µmol of bromide ions. Accordingly, the ratio between chloride and bromide ions was 8.5:1.

Photoluminescence measurements
The PL was measured in the FLSP920 fluorimeter spectrometer. The NCs were excited at 300 nm using Xe-900 lamp (Edinburgh Instruments). The spectra were measured continuously in a range of 400-540 nm with a 2 nm step with high speed photomultiplier tube R7400U (Hamamatsu) detector. The gap between the measurements was 18 seconds.
The C3-ASC18 and OA/OLA measurements were faster and required higher temporal resolution. In this case an additional setup for light collection was used. Following the NCs excitation, as mentioned before, the light was collected using an optical fiber QP400-2-VIS-NIR (Ocean Insight), with core size of 400 µm, and detected with Ocean Insight USB4000-UV-VIS-ES. The gap between measurements was 1 second.
A setup of lenses was used to enhance the signal (Scheme S1).
15 Scheme S1. Scheme of the setup used for light collection with an optical fiber and detection with a CCD detector. Focal length of lenses is 3 cm. 16

Anion exchange product
The final product of the cross-anion exchange reaction was further characterized by TEM and STEM. As presented in Figure S4, the NCs shape is preserved.

Initial rates method
According to the initial rates method, the order of the reaction can be determined under the assumption that at the beginning of the reaction, the rate depends on the initial concentration of the reactants. 7 Therefore, the reaction order is the slope of the plot of the logarithm of the initial rate versus the logarithm of the initial concentration, according to equation: .
Where is the order, and in this case, [Cl]0 is the initial concentration of chloride ions (which is equal to 8.5 times the initial concentration of bromide ions).
In order to extract the reaction order as described, several experiments were conducted at various initial halide concentrations, while keeping the total volume identical, and 8.5:1 ratio between chloride and bromide ions.
a. Concentration calculation: The initial concentration of chloride ions was calculated from the absorbance of the solution, which is proportional to the NCs concentration, with the consideration of the number of halides per NC (details in section 4).
b. Initial rate determination:

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The initial rate was determined using a linear fit on the initial portion of the data.
The slope is the initial rate. a. Tube cleaning: As-purchased tubes are in a solution of 0.05% sodium azide in water. Since moisture is highly harmful to perovskite NCs, a cleaning procedure was necessary prior to the cross-anion exchange experiments.
The tube was submerged in a solution of 15% ethanol in 1-propanol, and heated to 40 ⁰C for 20 minutes. Then 5 ml of the solvent mixture was poured into the inner side of the tube. This step was repeated three times, and then three additional times with toluene at 60 ⁰C.
b. Experimental setup: Control experiment: In order to confirm that the NCs cannot pass through the pores, we conducted a control experiment, in which a tube containing CsPbBr3 NCs solution was immersed in pure toluene. After 3 hours the PL of the toluene 21 was measured and revealed no significant signal, which means that CsPbBr3 NCs did not migrate through the membrane pores ( Figure S7).  However, the influence of the surface capping ligand is significantly stronger. Figure   S12a presents the rate constants of the three systems, differing by the capping ligand, with similar sized NCs. According to the received trend, lecithin is the most stabilizing ligand and OA/OLA is the least stabilizing ligand over the studied size range. Figure S12b shows that the activation energy almost does not change with variation in the NC size, within the tested range.

Mechanism
The investigated cross-anion exchange reaction can be written as: Reaction S1. 0.9 3 + 0.1 3 → ( 0.9 0.1 ) 3 Where a stoichiometric ratio of 8.5:1 chloride to bromide anions is considered. For simplicity, we refer to CsPbCl3 and CsPbBr3 NCs as (NC)Cl and (NC)Br, respectively.
For either pure NCs solution or during the cross-anion exchange, a ligand association and dissociation process is assumed to be in a pre-equilibrium condition: As the experiment cannot distinguish between sites that are bound to a ligand and sites that are not in the following reactions, NC-X describes a general surface site (X=Cl, Br).
The cage effect occurs between a surface site and a halide bound ligand The rate of the reaction can be expressed using the rate of product formation: By using Steady state approximation on the concentration of the caged complex: .
the concentration of the caged complex is: The rate of the reaction, after plugging eq. (S10) into eq. (S8) is: Considering the constant concentration of the free ligands, the reaction is a pseudo first order reaction with respect to the halide concentration.
14. Viscosity measurements Viscosities measurements were performed using Brookfield DV-II viscometer. (100 RPM).   Exponential fitting to the difference between the energy at the emission peak and the energy at equilibrium versus time, with varying ratios of lecithin per Cs surface sites.